1. Field of the Invention
The present invention relates to a motor speed controller for suppressing the resonance generated in the driving of a load.
2. Description of the Prior Art
It is known in the art to insert a notch filter in front of the torque controlling unit of a motor speed controller in order to suppress resonance. An example of such a system can be found in Japanese Patent Publication No. 21478, as shown in prior art FIG. 11.
In FIG. 11, motor 1 drives a load 4 via coupler 3. The torque of the motor 1 is controlled by torque controlling unit 8, which receives a new torque command .tau..sub.2 * from a notch filter 7. The notch filter 7 is used to filter a torque command .tau..sub.2 * output from operating/amplifying unit 6 so that any resonance appearing in the signal can be eliminated to produce the new torque command .tau..sub.2 *. The operating/amplifying unit 6 outputs the torque command .tau..sub.1 * as a deviation value .omega..sub.r *-.omega..sub.r, representing the difference between the speed command value .omega..sub.r * and a speed detection value .omega..sub.r. Subtractor unit 5 receives the speed command value .omega..sub.r * from the machine controller (not shown) and the detected speed .omega..sub.r from speed detector 2.
When entered into the subtracting unit 5, the speed detection value .omega..sub.r output by the speed detector 2 is fed back to the motor speed controller. Then the operating/amplifying unit 6 operates and amplifies the deviation value .omega..sub.r *-.omega..sub.r entered from the subtracting unit 5 until the value is zero. The unit 6 outputs the result as the torque command value .tau..sub.1 *, which is then input to the torque controlling unit 8 via notch filter 7, and the motor 1 is speed-controlled to follow the speed command value .tau..sub.r * by the output signal of the torque controlling unit 8.
FIGS. 12(a) and 12(b) illustrate the transfer functions of the torque controlling unit 8 and motor 1. The figures indicate the relationship between frequency and gain, and between frequency and phase, respectively, wherein the peak of the gain exists at the frequency f.sub.p due to machine resonance.
FIGS. 13(a) and 13(b) illustrate the transfer functions of the notch filter 7 having a central frequency f.sub.c.
FIGS. 14(a) and 14(b) illustrate the functions after passing through the notch filter 7 to the motor 1 when the resonance frequency f.sub.p and the central frequency f.sub.c of the notch filter 7 have been adjusted to match each other. As shown in FIG. 14(a), the peak of the gain due to the machine resonance in FIG. 12(a) and the notch of the gain of the notch filter 7 in FIG. 13(a) offset each other to suppress the machine resonance, thus eliminating the peak from the gain characteristic. As a result, the gain of the operating/amplifying unit 6 can be raised to enhance the response of the speed control system. However, the adjusted filter coefficient is fixed. Hence, if the machine resonance frequency f.sub.p changes according to load fluctuations, machine variations, operating environment changes, deterioration with age, etc., the resonance frequency f.sub.p will not match the central frequency f.sub.c of the notch filter 7. Thus, the suppression of the resonance may not be achieved and the motor control system may become unstable.
A phenomenon occurring due to a mismatch of the notch filter 7 central frequency f.sub.c and the machine resonance frequency f.sub.p will now be described.
FIGS. 15(a) and 15(b) illustrate the characteristics of transmission from the notch filter 7 to the motor 1 at a time when the notch filter 7 central frequency f.sub.c is lower than the machine resonance frequency f.sub.p. As shown therein, the peak of the resonance gain is not suppressed sufficiently and a phase delay value in the low frequency range is increased by the phase delay of the notch filter 7; thus, deteriorating the speed response characteristic, e.g., an overshoot increase.
FIGS. 16(a) and 16(b) illustrate the functions of the transmission from the notch filter 7 to the motor 1 at a time when the notch filter 7 central frequency f.sub.c is higher than the machine resonance frequency f.sub.p. As shown therein, the peak of the resonance gain is not suppressed sufficiently and the phase delay value in the neighborhood of the gain peak is greatly increased by the phase delay of the notch filter 7. This makes the speed control system unstable and may cause oscillation depending on the gain of the operating/amplifying unit 6, leading to a failure of control.
To avoid an unstable phenomenon occurring when the notch filter 7 central frequency f.sub.c is higher than the machine resonance frequency f.sub.p (as illustrated in FIG. 16(a)), it is inevitable for the motor speed controller of the prior art to take the variation of the machine resonance frequency f.sub.p into consideration and set the notch filter 7 central frequency f.sub.c to be lower than the machine resonance frequency f.sub.p. However, this results in the deterioration of the speed response characteristic.
In the motor speed controller according to the prior art, the filter coefficient defining the characteristic of the notch filter 7 for suppressing the machine resonance may be manually adjusted by an operator on a machine-by-machine basis using an oscilloscope, an FFT analyzer, etc., so that the central frequency f.sub.c of the notch filter 7 matches the machine resonance frequency f.sub.p. This is done in accordance with the speed detection value .omega..sub.r of the motor 1 at a time when the speed command value .omega..sub.r * is provided from an external oscillator, or the like. However, this adjustment has the disadvantage that it requires measuring instruments, such as an oscillator and an oscilloscope, as well as much time and skill.
In addition, since the adjusted filter coefficient is fixed, if the machine resonance frequency f.sub.p changes according to load fluctuations, machine variations, operating environment changes, deterioration with age, etc., and as a result, does not match the central frequency f.sub.c of the notch filter 7, the critical problems mentioned above may arise.
A motor control circuit is disclosed in Japanese Patent Disclosure Publication No. 46184, wherein the resonance suppression circuit described above is applied to the take-up mechanism of a magnetic tape storage device. When a tape is taken up, the resonance filter of the mechanical system varies according to the take-up position (value) of the tape. The control circuit is designed to overcome the disadvantage caused by a mismatch of the machine resonance frequency and a notch filter central frequency by changing the central frequency in three stages in accordance with an external signal corresponding to the changes of the machine resonance frequency. The central frequency is changed by switching a resistance element, comprising the notch filter, using a short-circuit switch.
As the machine resonance frequency changes continuously according to the take-up value of the tape, a problem arises in that a mismatch of the machine resonance frequency and the notch filter central frequency will occur which cannot be prevented by merely changing the notch filter central frequency in three steps, as indicated in the embodiment.
A positioning controller disclosed in the Japanese Patent Disclosure Publication No. 64599 is employed to position the magnetic head of a disc drive using a step motor. In advance of ordinary access to a disc, the positioning controller first detects the vibration of the magnetic head at a time when the magnetic head is moved step by step to vibration detecting tracks provided at specific positions of the inner and outer peripheries of the disc, to select a notch filter on a trial and error basis. This filter will suppress the amplitude of that vibration within a predetermined value from among a plurality of prepared notch filters different in frequency characteristic. During ordinary access to the disc, the vibration characteristic of the step motor varying according to the track position is suppressed by selecting the appropriate notch filter for the tracks divided into three areas: inner periphery, middle and outer periphery. That is, the notch filter selected by the inner periphery vibration detecting track is chosen on the inner peripheral tracks, the notch filter selected by the outer periphery vibration detecting track is chosen on the outer peripheral tracks, and the notch filter representing the characteristic midway between the inner periphery notch filter and the outer periphery notch filter is chosen for the middle tracks.
In this manner, it is necessary to select the notch filter that meets the step motor vibration characteristic changing in accordance with the magnetic head position and the magnetic disc installed. This creates a problem in that it requires a plurality of notch filters to be prepared, making the circuit larger in scale and complicated in structure. The circuit structure is further complicated by a step response detecting unit which must be added to detect the vibration of a moved member, i.e., the magnetic head.